Method for thermal decomposition of plastic waste and/or biomass and apparatus for process management

ABSTRACT

There is presented a method for thermal decomposition of plastic waste and/or biomass, characterized by that fact the plastic waste and/or biomass are subjected to a temperature in a reactor in the presence of loose three-dimensional elements of a developed surface area, resistant to the process heat. The invention also involves an installation to carry out the process.

The subject of the invention is a method for thermal decomposition of plastic waste and/or biomass and an installation for the process.

Among various processes of utilization of plastic waste, pyrolysis processes are considered the most desirable as they lead to obtaining useful chemical substances such as pyrolytic gases and oils, as well as production of heat. Another advantage of the pyrolysis processes is that, in principle, they do not require that raw materials be separated by type of contained polymer. The plastic waste being processed can also be contaminated with e.g. different types of biomass.

Processes of thermal decomposition also include e.g. processes of thermo-catalytic depolymerisation or gasification.

The main requirements to be met when developing a technology for conducting processes of pyrolysis are, in particular, the continuity of the process, ensuring an adequate quality of obtained products and providing a convenient method for removing post-processing pollution from the reaction area.

The basic problem in processes of depolymerisation of plastics waste is a very low thermal conductivity coefficient and, therefore, difficulties in supplying heat to the processed material in a manner ensuring its uniform distribution throughout the mass of the substrate. Most frequently, it leads to local overheating of the reaction mass and excessive precipitation of elemental carbon firmly adhering to the heating surfaces of the reactor. This excessive process of carbonization of the raw material generates unnecessary disruptions in the process, as well as losses in amounts of obtained products and too much non-condensable gas, which altogether result in a dramatic decrease in the economic efficiency of the entire project.

There are known pyrolysis processes in which the risk of creation of biochar has been reduced by using spent mineral oil as a “solvent” of plastic; however, the problem of this technology is a high degree of sulphation of products of depolymerisation as sulphur is an essential ingredient of lubricating mineral oils.

During the use of pyrolysis processes, a wide fraction of products is received (KTS-f), which is a mixture of various kinds of hydrocarbons with a carbon chain length of C2 to C35, including three essential fractions in terms of the distillation curve. These are: 1) a paraffinic fraction boiling, in a temperature range from 450 to 360° C. with a flow temperature of over 40° C., 2) an oil fraction with a boiling range from 180 to 360° C.; this is the most desired fraction, which, when hydrogenated, is a product comparable to light fuel oil or diesel oil, 3) a naphtha fraction with a boiling range from 30 to 180° C.; this is a fraction of a very low flash point, which could be a component of heavy naphtha 4) a fraction non-condensing under normal conditions, namely post-processing gas most often used as a source of energy used to heat reactors in the process of depolymerisation of plastics.

The paraffin fraction is the most problematic for manufacturers of KTS-f because it gives the whole mixture of KST-f the character of a solid body even at 30° C.

For the above reasons, improved pyrolysis processes are still sought so as to minimize the biochar formation and to depolymerize the paraffin fraction in order to minimize formation of this fraction in the process of processing plastic waste. There is also a need for methods for continuous processes of degradation of plastics.

The subject of the invention is a method for thermal decomposition of plastic waste and/or biomass, characterized by that fact that the plastic waste and/or biomass are subjected to a temperature in a reactor in the presence of loose three-dimensional elements of a developed surface area, resistant to the process heat.

The ratio of the surface area of the three-dimensional elements to the mass of plastics and/or biomass subjected to degradation is from 25 m² per 1000 kg of the raw material to 600 m² per 1000 kg of the raw material

In a reactor in which the process of thermal decomposition of plastic waste and/or biomass occurs, there is a temperature gradient within the range from 450 to 550° C. in the vertical axis of the reactor, preferably 500° C. at the base and decreasing gradually to a temperature within the range from 320 to 400° C., preferably 360° C. at the top.

Three-dimensional elements are fed into the reactor all at once or continuously or in portions, and plastic waste and/or biomass is fed into the reactor continuously or in portions.

Three-dimensional elements are supplier to the reactor before it is fed with plastics waste and/or biomass, or a mixture of three-dimensional elements and plastic waste and/or biomass is provided to the reactor.

Three-dimensional elements are made of metal or are ceramic.

Contaminated three-dimensional elements are removed from the reactor continuously or in portions.

The subject of the invention is also an installation for the process of thermal decomposition of plastic waste and/or biomass, consisting of raw material storage containers, a raw material preparation section, a reactor, means heating the reactor, means of transport of raw materials to the reactor and a section for the discharge of products of thermal decomposition of plastic waste and/or biomass, characterized by the fact that the reactor is at least partially filled with loose three-dimensional elements of a developed surface area, resistant to the process temperature.

The reactor contains such a number of three-dimensional elements that the ratio of the surface area of the three-dimensional elements to the mass of plastics and/or biomass subjected to degradation in the reactor is from 25 m² per 1000 kg of the raw material to 600 m² per 1000 kg of the raw material.

The three-dimensional elements contained in the reactor are made of metal or are ceramic.

The method and installation according to the invention enable elimination of the adhesion of biochar to the walls of heating surfaces of the reactor and, generally, to minimize the amount of forming biochar by mitigating the temperature of the process. The heat exchange surface between a donor of thermal energy and the raw material being depolymerized is also increased. The method used to carry out the process leads to depolymerisation of the forming undesired paraffin (heavy-fractions of the depolymerizer) in a single technological process. There is provided a trouble-free and continuous method for removing post-processing residues from the reactor. The process according to the invention leads to obtaining products with a distillation temperature of over 360° C. exclusively.

In processes of gasification of various substances, especially waste, in particular plastic waste or biomass, the problem of heat conduction in the reaction space is encountered. Substances subjected to heat treatment are characterized by a low thermal conductivity coefficient; therefore, to heat them to an appropriate temperature in industrial processes, the use of agitators or moving reactors is required. It is usually impossible to fully solve the problem of removing post-processing remains from the reaction area and the issue of adhesion of biochar to the heating surfaces of the reactor. Moreover, in processes of thermo-degradation of plastics or biomass, there is the problem of disposing of post-processing waste such as mineral impurities or heavy tar from the reaction area. Biochar formed in the process must be removed periodically from the installation, which requires interrupting the process and is time consuming. Some components of the installation may be irremediably damaged.

At present, the above-described problems still can be observed with conventional methods of heating raw materials (conversion or conduction) in thermal decomposition processes such as pyrolysis, thermo-catalytic depolymerisation or biomass gasification

In the process of developing this invention, the said concerns and properties of materials subjected to the mentioned processes were considered, and emphasis was placed on the following factors: 1) a thermal conductivity coefficient in the reaction mass to be processed, 2) an infrared radiation absorption coefficient in these raw materials, and 3) an infrared radiation reflection (emissivity) coefficient.

The thermal conduction coefficient in raw materials such as plastic is at a very low level and is in the range between 0.18 and 0.4 W/mK, and in dry biomass—between 0.12 and 0.24 W/mK. The thermal conductivity coefficient at such a level causes that the surface of a body being heated is hot and becomes charred, but its interior still may be cold. Therefore, it is a favourable solution to heat raw material particles of a small size and thickness, such as sawdust or woodchips, or regrinds or flakes/chips in the case of plastic, preventing them from fusing into a solid mass.

On the other hand, it has been noticed that these materials have a high infrared radiation absorption coefficient, which for plastics amounts to from 0.86 to 0.95; similar values can be observed for biomass or wood.

For the present invention, it is crucial to note that some metals are characterized by a very low infrared radiation emissivity coefficient. Polished gold has the lowest emissivity coefficient, but in the case of industrial applications, polished aluminium is most commonly used.

In a figure below, the subject of the invention is shown as a sample embodiment.

FIG. 1 presents a sample structure of a vertical, cylindrical reactor, which is a component of the installation according to the invention to carry out depolymerisation processes of plastic waste and/or biomass.

FIG. 2 shows a sample block diagram of the installation to carry out the process of thermal decomposition of plastic waste and/or biomass by the method according to the invention.

FIG. 3 shows a reactor designed to carry out a continuous process of thermal decomposition of plastic waste and/or biomass. The reactor is equipped with a worm feeder transporting plastic waste to the reactor and a worm conveyor carrying the filling in the form of solid elements contaminated with biochar, along with other unreacted remains, away from the reactor.

FIG. 4 presents an explanation of the graphic signs used in FIG. 3.

In the sample embodiment, the installation according to the invention comprises a shredding section (1) consisting of various known devices grinding plastic waste into a fraction of a size of from 0.1 to 5000 square centimetres as regards foil and a fraction of from 0.1 to 50 mm in diameter as regards regrinds. The grinding section is optionally followed by a dryer (2) of ground plastic material, connected to the former with a conveying link; next to the dryer, there is a mixer (3) of dried raw material (the processed raw material is labelled with S in the figure) with a catalyst used optionally and with three-dimensional elements (labelled with E in the figure), jointly forming the filling of the reactor. The installation also comprises a feeder (4) transporting the raw material (S) mixed with the catalyst and the three-dimensional elements (E) to a loading container (5) of the reactor (6). Alternatively, the installation may not include the mixer of raw materials, but the components can be delivered alternately in batches to the reactor from a container (7) of three-dimensional elements, a container (8) of plastic waste and a container (9) of biomass, i.e. a portion of the three-dimensional elements is fed into the reactor first, and then the ground plastic waste, optionally mixed with the catalyst. Next, successively, three-dimensional elements (E) and ground plastic waste and/or biomass (S) can be supplied alternately or three-dimensional elements can be supplied on a one-off basis, and then raw materials of plastic waste are fed continuously or in batches. The installation can also be fitted with a buffer tank (10) of the reactor. The reactor is equipped with a heating system (11). The installation also includes a conveyor (12) with a cooling system to discharge the filling in the form of three-dimensional elements (E) from the reactor, along with biochar and impurities remaining after the depolymerisation process. The conveyor (12) is followed by a section for the cleaning (13) of three-dimensional elements, where it is possible to sift spent catalyst and biochar. Cleaned three-dimensional elements can be returned into the container (7) or directly to the reactor (6). The installation also includes a receiver (14) of the post-processing steam (products labelled as P in the figures) from the reactor and a condenser (15) of the post-processing steam, as well as a container (16) for the condensate. The reactor walls are covered with lining (17) on the inside, i.e. with a material of low emissivity, preferably polished aluminium.

Most of the stages of the plastic depolymerisation process are known as the state of the art. The method applies such known stages as preparation of the raw material, including grinding and drying, mixing raw materials of various types of plastic optionally with a catalyst and optionally with biomass, transport of the raw material between the grinding and drying sections, heating the reaction mixture in the reactor leading to depolymerisation of the processed raw materials, discharge of the post-processing steam and discharge of post-processing impurities.

The method according to the invention applies a new stage consisting in introducing loose three-dimensional elements, constituting profiled movable filling of the reactor, into the reaction space. The term that these elements are loose means that they are not fused in any way with the reactor, but are elements that can move freely during the process, i.e. when raw materials are processed, melt and decompose, these elements slide down and change their spatial arrangement. Three-dimensional elements (profiles) have a different form and are characterized by a high ratio of the surface area to their bulk volume, as well as a large throughput as regards gases. The three-dimensional elements are also characterized by a low infrared radiation emissivity coefficient. Favourably, the shape of three-dimensional elements allows them to achieve the surface area of 25 to 600 m² per 1 m³ of bulk mass of these elements, preferably from 60 440 m² per 1 m³. The outer diameter of ring elements and, generally, the size of the three-dimensional elements is of 1 to 100 mm, preferably from 15 to 25 mm, with the wail thickness is of 1.5 to 10 mm. The profiled elements are made of materials resistant to the reaction temperature, e.g. metal, glass, ceramics.

Preferably, they are resistant to temperatures of 450 to 1650° C. The three-dimensional elements may have different shapes e.g. they can be in a form of rings with or without holes in the side walls or a form of spatial forms woven of longitudinal elements such as wires or metal ribbons. They may take the form of a wave, a sphere sector or be in the shape of letters C, V, M, N or S, formed of a metal ribbon. Three-dimensional elements of this type are known from processes of distillation, rectification or aeration, where their role is to increase the area of contact between different phases of a substance, e.g. liquid-gas (distillation columns, bioreactors), liquid-solid (dusts-water in filters), gas-gas (in various types of reactors of chemical synthesis). Typical examples of this kind of three-dimensional elements constituting a filling of reaction columns are well-known Raschig rings, Pall rings, Bial/ecki rings and others.

Surprisingly, it has been found that this kind of three-dimensional elements (profiles) facilitate the depolymerisation process (of thermal decomposition, pyrolysis).

Favourably, in the method according to this invention, the ground mass of plastic with the fragmentation range from 0.1 mm to 10 mm or, in the case of film waste, with the flake surface area, of up to 100 cm², is mixed with three-dimensional elements (profiles), preferably in the form of rings of different diameters, preferably in the range between 3 to 50 mm, of a material with the lowest possible coefficient of emissivity, preferably of aluminium. Plastic is mixed with three-dimensional elements in various mass ratios, preferably with an excess of three-dimensional elements, i.e. in a ratio of 1:5 to 1:1. The mixed mass of ground plastic, optionally with biomass, is fed into the reactor, preferably a cylindrical one, equipped with internal heating elements, preferably infrared radiators located inside the reactor, preferably radially, as shown in FIG. 1. The walls of the reactor are preferably made of a material with low emissivity or covered with lining on the inside, i.e. with a material of low emissivity, preferably polished aluminium.

The method according to the invention can be implemented in several variants. The most advantageous one is to carry out the process continuously by feeding the reactor continuously with a mixture of three-dimensional elements and plastic, optionally combined with biomass and a catalyst, or by supplying it continuously with alternating portions of three-dimensional elements and the raw material being processed. In another variant, the process may not be carried out continuously, but the reactor may be filled with three-dimensional elements and then filled with plastic being treated and/or biomass, either in portions or continuously. Regardless of the selected variant of the process, it is preferred to fill the reactor, at least partially, with three-dimensional elements before feeding it with plastic waste.

A reactor for depolymerisation can be of a cylindrical or another shape, e.g. cuboidal. For the purpose of the continuous process, the reactor is preferably of a cylindrical shape with a diameter of not less than 200 mm and a height being a multiple of the diameter; it is best when the diameter to height ratio is 1:6. Optimal dimensions of a reactor for industrial production of the capacity of 1400 kg per 1 hour are: diameter—1800 mm and height—10000 mm. The reactor is heated diaphragmatically or is fitted with heating elements, preferably electric, preferably IR emitters arranged radially inside the reactor on multiple levels. For example, heating elements are arranged on the circumference of the reactor in a vertical plane from the bottom to the top of the reactor at a distance of 20 cm so that the amount of heat provided by all the heaters is equal to or greater than 0.3 kW per 1 kg of the raw material supplied to the reactor in the form of ground plastic waste or biomass per 1 hour.

The reactor may also be heated from the bottom, especially when the conducted process is not a continuous process, and the reactor is not equipped in the bottom part with a section receiving contaminated three-dimensional elements, along with biochar. When the process is carried out periodically (not continuously), the reactor may be, in principle, of any shape, and the heating sections can be arranged freely, but evenly on the bottom of the reactor and/or its side walls.

In the case where the process is carried out continuously, the raw material in a ground form mixed with profiled elements and, optionally, a depolymerisation catalyst is fed continuously into the reactor from the top, and it moves vertically down to a discharge outlet of the reactor, from where post-processing residues are removed, along with the filling of the reactor. When the filling of the reactor (movable profiles) moves down with the raw material under the influence of the supplied energy (heat), anaerobic gasification of the raw material takes place, and vapours of the depolymerizer is discharged via an interceptor to a condenser or directly to a distillation column, where the product is separated into appropriate fractions. During the process, the raw material is maintained in a loose spatial arrangement, which causes that the heat interacts with small and thin layers of plastic, e.g. in the form of a flake or regrind or biomass, e.g. in the form of woodchips, sawdust, chopped straw, etc. Profiled elements prevent the molten raw material from agglomerating into a homogeneous mass. Even in the phase when the plastic has already been liquefied, it does not flow down freely to the bottom of the reactor, but it spreads out on the large surface area of rings (three-dimensional elements filling the reactor), remaining in the form of a thin coating on the surface of the rings until complete decomposition and evaporation.

Three-dimensional elements contained in the reactor occupy at least ⅕ of its height, and when the reactor is fed with a mixture of profiled elements and raw materials to be treated, it is understood that the ratio of the volume of these elements to the volume of the raw material is such that during the process of falling down to the bottom, the three-dimensional elements ultimately occupy at least ⅕ of its height.

Thermal infrared radiation that does not reach directly the raw material being depolymerised or is not absorbed by the same or is re-emitted by the hot raw material is reflected from the elements filling the reactor or the lining of its walls and then re-absorbed by a portion of the raw material of a lower temperature or by a dripping condensate. This system ensures an even distribution of temperature throughout the entire reaction mass. Since the raw material is ground, it does not become locally overheated, and thus there is no excessive carbonization. Such a system allows one to carry out the gasification or depolymerisation process under mild conditions, while maintaining the optimum temperature of the heating surfaces and a low Δt, at a level that guarantees the flow of heat from the surface of the heating element to the raw material being processed between 30 and 50° C. The method involving continuous loading the reactor with a cold raw material from the top and receiving three-dimensional elements of the filling of the reactor, along with contaminants—post-processing residues and biochar, at the bottom of the reactor, causes a difference in temperatures at the top, middle and bottom of the reactor. Such a natural distribution of temperature in the reactor causes that heavier fractions of the depolymerizer condense on the colder raw material fed from the top and on elements of the filling of the reactor, and then drip down to the hotter zone of the reactor, where they are reheated and further depolymerized until reaching the desired distillation temperature. The process of repeated (cyclically) depolymerisation of high-boiling components of the depolymerizer takes place automatically, due to a natural distribution of temperature in the vertical axis of the reactor. The reactor is continuously fed with fresh portions of the raw material mixed with cold three-dimensional elements of the filling of the reactor. Thus, the temperature in the upper part of the reactor is lower than in the middle and bottom zone. This way, vapours of heavy fractions of the depolymerizer, a boiling point of which is higher than the temperature in the upper zone of the reactor, are condensed on the colder particles of the raw material and elements filling the reactor, soaking the raw material and flowing down to the hotter parts of the reactor where they are re-subjected to a higher temperature which results in the further process of depolymerisation. This means that long carbon chains are further divided (depolymerized) to shorter hydrocarbon compounds characterized by a lower boiling point allowing them to leave the reaction area.

Then, steam of the depolymerizer is collected in the area of the reactor where the temperature is about 360° C., characteristic of distillation of typical fuel oil. Thanks to using three-dimensional elements filling the reactor, the condensate of heavier fractions is cyclically and repeatedly heated, and the process of depolymerisation is continued until their boiling point falls below the temperature of 360° C., and they can no longer condense within the depolymerisation reactor, escaping with an inert gas to the collector and further to the condenser or the fractionating column where they are separated into fractions boiling at a temperature of up to 180° C. (naphtha) and fractions boiling at a temperature of up to 360° C. (oil fraction).

EXAMPLE 1

The process of thermal decomposition was applied to a mixture of plastic waste containing PE. The plastic was being ground until obtaining fragments of a size not exceeding 50 cm². The raw material was also mixed with a ZSM5 zeolite catalyst, free-flowing. The resulting mixture was combined and mixed with three-dimensional elements in the form of aluminium rings with a diameter of 12 mm and a length of 15 mm and a wall thickness of 1 mm. The mass ratio of the raw material being processed to the three-dimensional elements was 1:5. The process was carried out continuously at a temperature of 480° C. at the bottom of the reactor and 360° C. at the top of the reactor. The reactor was fed from the top with portions of the raw materials and the three-dimensional elements. In the lower part of the reactor, ring elements contaminated with biochar and unreacted residues were collected through the cooling section. There was obtained a clear liquid product of a characteristic smell and with the distillation curve shown in Table 1 below.

TABLE 1 Fractional composition Unit Test result Test method boiling begins ° C. 101.7 PN-EN ISO 3405 5% (m/m) distilling ° C. 126.6 10% (m/m) distilling ° C. 158.7 20% (m/m) distilling ° C. 190.3 30% (m/m) distilling ° C. 216.1 40% (m/m) distilling ° C. 240.9 50% (m/m) distilling ° C. 262.1 60% (m/m) distilling ° C. 282.2 70% (m/m) distilling ° C. 300.5 80% (m/m) distilling ° C. 317.7 90% (m/m) distilling ° C. 337.0 95% (m/m) distilling ° C. 349.7 end of distillation ° C. 372.5 up to 250° C. % (V/V) 44.2 distilling up to 350° C. % (V/V) 95.1 distilling the sample did not contain a depressant

EXAMPLE 2

In a similar manner and under similar conditions, a mixture of PP and PE (1:1 by weight) was treated, and the resulting ground mixture was supplemented with 5% (by weight) of biomass constituting cellulosic waste (recycled). Similar results were obtained with respect to the depolymerizer. 

1. Method for thermal decomposition of plastic waste and/or biomass, characterized by the fact that the plastic waste and/or biomass are subjected to a temperature in a reactor in the presence of loose three-dimensional elements of a developed surface area, resistant to the process heat.
 2. Method according to claim 1, characterized by the fact that the ratio of the surface area of three-dimensional elements to the mass of plastics and/or biomass subjected to degradation is 25 m² per 1000 kg of the raw material to 600 m² per 1000 kg of the raw material.
 3. Method according to claim 1, characterized by the fact that in a reactor in which the process of thermal decomposition of plastic waste and/or biomass occurs, there is a temperature gradient within the range from 450 to 550° C. in the vertical axis of the reactor, preferably 500° C. at the base and decreasing gradually to a temperature within the range from 320 to 400° C., preferably 360° C. at the top of the reactor.
 4. Method according to claim 1, characterized by the fact that three-dimensional elements are fed into the reactor all at once or continuously or in portions.
 5. Method according to claim 1, characterized by the fact that plastic waste and/or biomass are fed into the reactor continuously or in portions.
 6. Method according to claim 1, characterized by the fact that three-dimensional elements are supplied to the reactor before it is fed with plastics waste and/or biomass, or a mixture of three-dimensional elements and plastic waste and/or biomass is provided to the reactor.
 7. Method according to claim 1, characterized by the fact that three-dimensional elements are made of metal or are ceramic.
 8. Method according to claim 1, characterized by the fact that contaminated three-dimensional elements are removed from the reactor continuously or in portions.
 9. Installation for the process of thermal decomposition of plastic waste and/or biomass, consisting of raw material storage containers, a raw material preparation section, a reactor, means heating the reactor, means of transport of raw materials to the reactor and a section for the discharge of products of thermal decomposition of plastic waste and/or biomass, characterized by the fact that the reactor is at least partially filled with loose three-dimensional elements of a developed surface area, resistant to the process temperature.
 10. Installation according to claim 9, characterized by the fact that the reactor contains such a number of three-dimensional elements that the ratio of the surface area of the three-dimensional elements to the mass of plastics and/or biomass subjected to degradation in the reactor is from 25 m² per 1000 kg of the raw material to 600 m² per 1000 kg of the raw material.
 11. Installation according to claim 9, characterized by the fact that three-dimensional elements in the reactor are made of metal or are ceramic. 